1. Field of the Invention
The present technology relates to calibration of storage tanks. In particular, the present technology relates to calibration of storage tanks by measuring the horizontal offset of the wall of a tank relative to a vertical laser reference line.
2. Description of the Related Art
Over time, the price of oil and gas products has increased. As a result, the accurate measurement of oil and gas in storage has become increasingly important. Typically, oil and gas can be stored in tanks, many of which are extremely large (e.g., up to about 2,000,000 barrels in volume or more). Accurate knowledge of the volume of such tanks is important so that the owner can maintain accurate information about the amount of oil and gas in the tanks.
There are a number of methods of calibrating, or measuring the volume of these large tanks. For example, one method is to fill the tank, then meter the liquid as the tank is drained to determine the capacity of the tank. This method, however, is very time consuming, and can be very costly because of the size of the tanks. Normally, this method is avoided unless the tank volume cannot be determined geometrically through physical measurement of the tank parameters.
Another method for calibrating tanks is called the optical reference line method (ORLM). The ORLM provides for the calibration of cylindrical tanks by measurement of one reference circumference, followed by determining the remaining circumferences at different elevation levels on the tank. The remaining circumferences are determined by measuring the horizontal offset of the tank wall from a vertical optical reference line. These circumferences are corrected, based on wall thickness, to calculate true internal circumferences, which can then be added to determine the tank volume.
An example of the ORLM method is shown in FIG. 1, in which there is shown a tank 2, a magnetic trolley 4, an optical device 6, and a horizontal graduated scale 8 attached to the trolley 4. The optical device 6 produced an optical ray of light 10 upwardly and parallel to the tank wall 12. The magnetic trolley 4 is typically controlled by an operator 11 positioned on top of the tank 2, that holds a rope 13 attached to the trolley. The operator 11 raises and lowers the trolley 4 on the tank wall 12 by manipulating the rope 13.
To measure the volume of the tank 2, a reference circumference C is first measured. The reference circumference is measured using a master tape (not shown), and is typically measured near the bottom of the tank 2. With the reference circumference known, the trolley 4 can be raised or lowered by the rope 13 to various vertical stations, or predetermined locations, along the tank wall 12. In most systems, the vertical stations are located between the weld seams on the tank. In FIG. 1, two of the vertical stations are indicated by lines V. At each vertical station V, the horizontal offset between the tank wall 12 and the optical ray of light 10 is noted, using the horizontal graduated scale 8. Once a series of measurements have been taken at the vertical stations V, the measurements are repeated with the optical device 6 rotated 180 degrees to verify accuracy. Thereafter the measurements are used to determine the circumference of the tank at each vertical station (using the reference circumference as a reference point), and the volume of the tank can be estimated. Additional factors can also be considered when calculating volume, such as, for example, the temperature of the tank wall 12. This temperature is typically derived based on the temperature inside the tank and the ambient temperature.
While the ORLM method shown in FIG. 1 is better in some ways than filling the tank and metering the fluid, as discussed above, it still has significant problems. For example, measuring the horizontal offset of the trolley 4 from the optical ray 10 at only a few select vertical stations V provides relatively few data points from which tank circumferences can be measured. Although this data can be extrapolated to estimate the volume of the tank, such extrapolations may not always be accurate. In addition, the method of FIG. 1 requires the operator 11 to be positioned on the top of the tank, which can be dangerous. Furthermore, the use of an optical ray 10 and a horizontal graduated scale 8 to measure the horizontal offset of the tank wall 12 lacks the precision necessary to calculate accurate tank volumes. This is because an operator must read the horizontal graduated scale 8 at each horizontal offset, often from a distance.
Another problem with known ORLM methods occurs when the storage tank has a protrusion 15 extending radially outward from the tank wall, which frequently occurs, and which is shown in FIG. 2. In such an instance, the ability of the operator 11 to raise the trolley 4 to the top of the tank 2 is restricted because the rope 13 has to be routed over the protrusion. When this happens, horizontal offset measurements cannot be made at the top of the tank, and in some instances the inaccuracies introduced into the volume calculations by the missing measurements can be great enough to render the ORLM calibration method unreliable.
What is needed therefore, is a tank calibration system that overcomes the disadvantages of known systems.